I think it boils down to two different concepts..
First, it's the "WHY DON'T YOU LIKE WHAT I LIKE!?" syndrome.... this is something that we all see even without Apple or iPhones in the mix. Someone likes device A, another one doesn't like device A and thinks its junk. So the first person takes it as a personal insult that anyone would dare to criticize a phone that the Almighty would be pleased with.... and an argument ensues. The larger the divergence between the two devices, the higher the chances of a drag-out row. So what is a mild disagreement between someone with a S6 and a Note 5 turns into a monkey-crap-fight if that Note 5 is replaced with a iPhone 6S.
The second part is more nuanced. And I place the blame for this one on Apple's shoulders. Early on, the iPhone was clearly ahead of any Android phone. The hardware, operating system and apps all heavily favored Apple's stuff. And in their ads, keynotes, etc... they (Apple) basically looked upon any Android device with disdain, almost insulted that anyone would even think about choosing such an inferior product over an iPhone. That dismissive disgust quickly filtered down to iPhone users, with the complete blessing of Apple, and it really got under the skin of the Android fans at the time.... kind of simmered and stewed a bit.
And all the while, Android improved. The OS made huge gains in usability, performance and reliability and the hardware it ran on advanced with leaps and bounds. Once the playing field leveled, all that frustration from earlier kind of boiled over and Android zealots, now-armed with the knowledge that the iPhone was no longer the superior device/ecosystem it once was, unloaded on the hard core Apple fans, most of which still held on to the outdated notion that their beloved platform is superior and to say otherwise is silly... a belief that
Apple keeps feeding to this day even though the once peerless iPhone is now the one playing catch-up behind the likes of the Nexus 6P, Note 5, etc...
So that leaves us with the digital Hatfield/McCoy blood fued that we have today.